A friend recently related the life of Robert E. Howard in a really fascinating way, and one of the points he made about how he got into writing Conan really stuck with me.
Howard started out writing historical fiction, but the research was pretty grueling and the outcomes mediocre, so he switched to a sort of adventurous self-insert character in historical fantasy settings, which later turned into Conan.
When I consider notable science fiction authors, they all seem to start with some understanding of a facet of the natural universe, then tease ideas into explorations of those points. To the layman on reflection, this might seem like some sort of predictive exercise, but I see it as a sort of rote process of extrapolation. Contrast that to science fantasy authors who focus on theme and aesthetic, imagining Westerns in future-space with gadgets or contemporary plots in scifi-associated settings, and there's quite a vast and exciting landscape left to be explored.